The present invention relates to thermally conductive materials and, more particularly, to thermally conductive interface material for heat generating devices, such as microprocessor power supply assemblies, that facilitate heat transfer from the heat generating device to a heat sink.
With increasing market pressure for smaller, faster, and more sophisticated end products using integrated circuits, the electronics industry has responded by developing large scale integrated circuit devices which occupy less volume, yet operate at high current densities. Such devices generate considerable heat during operation. If the heat is not adequately removed, the increased temperatures generated by the device will result in its failure.
A heat sink is commonly used to transfer the heat away from the device. The heat sink generally includes a plate or body formed from a conductive metal, which is maintained in thermal contact with the device for dissipating heat in an efficient manner. Fins optionally protrude from the plate for providing an increased surface area for heat dissipation to the surrounding environment.
The current industry technique for providing thermal contact between heat-generating electronic device and a heat sink is to interpose a thermal interface material between the two, which facilitates heat transfer from the device to the heat sink.
One method is to apply a ceramic filled thermal grease, which is typically silicone-based, between the heat sink and the power supply. Thermal greases provide excellent thermal properties, but require an extensive assembly process with high manufacturing cost. The product is usually applied by hand from a syringe or with an aluminum carrier. This process is labor intensive and slow and does not lend itself to automation.
Another method for providing a conductive interface includes the use of thermally-conductive wax compounds. These, however, are generally brittle at ambient temperatures and are easily chipped off, resulting in a high thermal resistance. The low viscosity of the wax at operating temperature causes it to flow out, resulting in a high thermal resistance. Further, because of the brittle nature of the wax compounds, they are difficult to manufacture and apply to a heat sink.
Thermally conductive silicone rubbers have also been used for conductive interfaces. Although soft and pliable, their silicone rubber requires relatively high pressure and a long warm up to attain a low thermal resistance. The rubber has poor flow characteristics, which results in a low thermal conduction when there is a mismatch of flatness between the heat sink and the heat producing device. Differences in the thermal coefficient of expansion between the silicone rubber and the heat sink can result in high thermal resistance during temperature cycling. These effects lead to a poor thermal conductivity from the heat-producing device to the heat sink. Still other thermal interfaces employ polymeric thermally-conductive cure-in-place compounds. These compounds are generally rigid after cure. They have a poor reliability because of a difference between the material and the heat sink, causing cracks and failure during temperature cycling. The polymeric materials are labor intensive to apply and require long cure times.
The present invention provides for a new and improved thermal interface which overcomes the above-referenced problems and other problems as well.
In accordance with the present invention, there is provided a novel thermal interface composition which undergoes a viscoelastic change at microprocessor operating temperatures to transfer heat generated by a heat source to a heat sink, the composition comprising:
(A) a viscoelastic composition which melts at a temperature within the range of the the operating temperature of the heat source, the viscoelastic composition comprising:
(1) a thermoplastic elastomer,
(2) an oil compatible with the elastomer, and
(3) a tackifying resin;
(B) a dispersing agent; and
(C) a thermally conductive filler dispersed within the viscoelastic composition.
The invention includes a method for making the novel thermal interface composition.